1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with the implementation of musical effects which change as a function of the musical octave.
2. Description of the Prior Art
Electronic keyboard operated musical instruments without the addition of certain auxiliary systems tend to generate each tone with a mechanical-like precision and sameness in response to each actuated keyboard switch. This precision and similarity of tone is in sharp contrast with tones produced by conventional orchestral acoustic musical instruments. Almost none of these acoustic musical instruments creates all its tones in a precise replica of each other as the fundamental pitch changes. Even a mechanical tone generator such as a pipe organ is carefully designed so that each individual pipe in a rank of pipes has its own individual attack/decay/release envelope as well as its own individual loudness and characteristic tone. Each of these individual characteristics is different for each pipe in the rank of pipes.
Several auxiliary subsystems have been incorporated into the implementation of electronic musical instruments to vary the generated tone character over the pitch range associated with the keyboard. In U.S. Pat. No. 3,610,805 entitled "Attack And Decay System For A Digital Organ" a system is disclosed in which the timing of the attack and release phases of an ADSR (attack/decay/sustain/release) envelope modulation function are varied by counting the number of half-cycles of the fundamental period of the corresponding generated musical tone. In this fashion each note on the keyboard is generated with its own individual ADSR envelope timing. The disclosed system has a negative characteristic attribute caused by a timing logic such that when the ADSR timing is adjusted for the middle range of the keyboard it is perceived that the low octave notes have a very slow and unnatural sluggish attack and release while the high octave notes have such a fast attack that the tone appears to be purely percussive with almost no perseptive attack phase.
In U.S. Pat. No. 4,214,503 entitled "Electronic Musical Instrument With Automatic Loudness Compensation" a system is disclosed for producing a constant loudness level for all generated notes by automatically applying an ADSR envelope level compensation for equalizing the frequency sensitivity of hearing as quantified by the Fletcher-Munson constant loudness curves.
Organs designed to be used in the rendition of classical and liturgical music generally contain stops having the generic name of "mixtures." Mixtures are a heritage derived from early pipe organ design. The mixtures were employed originally to provide higher harmonics to be added to pipes limited to diapason type tones which are characterized by a fairly limited number of harmonics in their tonal structure. The introduction of mixture stops to extend the limited harmonic structure of diapason tones occurred before organ pipe designers learned techniques for producing sounds with extended harmonics for pipes in the string and reed families of tones.
A mixture stop generally consists of three or four ranks of pipes which all have elements that sound simultaneously with each actuated keyboard switch. The mixtures are usually constituted from ranks of diapason type voices. A typical mixture tone changes in each octave by selecting combinations of pipe footages. An example of such of the footages of ranks combined for each selected keyboard range is
C.sub.2 -B.sub.2 : 2', 1 3/5', 1' PA1 C.sub.3 -B.sub.3 : 22/3', 2', 1 3/5' PA1 C.sub.4 -B.sub.4 : 4', 22/3', 2' PA1 C.sub.5 -C.sub.7 : 4', 22/3'.
Electronic organs usually implement mixture stops by a system of either mechanical or electrical unification of a single diapason voice tone generators. This method of unification to produce a mixture stop produces somewhat objectionable temperament beats which result from the unification technique of actuating the nearest true musical notes for the mutation footages of 22/3' and 1 3/5' instead of generating the tones of the true third and fifth harmonic of the actuated keyboard tone switch.